btConvex2dShape debug drawing fix (2.87 stable)

[ruby0x1]

The convex2d shapes don't draw their child shape as expected in 2.87 stable.

Inside btCollisionWorld.cpp in the debugDrawObject function, the COMPOUND_SHAPE_PROXYTYPE is handling it's children, so I added an else which draws them for the CONVEX_2D_SHAPE_PROXYTYPE as well.

} else if (shape->getShapeType() == CONVEX_2D_SHAPE_PROXYTYPE)
{
    const btConvex2dShape* convex2dShape = static_cast<const btConvex2dShape*>(shape);
    const btCollisionShape* childShape = convex2dShape->getChildShape();
    debugDrawObject(worldTransform,childShape,color);
} else

After fixing it, wireframes correctly show up for these shapes.
I would PR but I'm pressed for time and file an issue to remind myself.
Feel free to fix it if this is the correct/intended approach!

[bilenkonito]

This is still a problem in 2.88. Can we please get an official response?